1. Field of the Invention
This invention relates to a process and apparatus for eliminating dioxins which are discharged through incinerator chimneys and are regarded as hazardous to the health of animals and humans, hence are of world and national concern.
2. Description of the Prior Art
The chimneys of refuse incinerator plants, where packaging materials and other articles made of vinyl or like plastic materials, which are petrochemical products, are incinerated, together with domestic refuse, may discharge dioxin-containing smoke.
When heated in refuse incinerators, vinyl chloride-based plastics in the form of refuse soften and then begin to melt at 300xc2x0 C. to 400xc2x0 C. and, at higher temperatures, the combustion thereof begins. The combustion temperature further rises and arrives at about 800xc2x0 C., which is approximately the upper limit.
At 300xc2x0 C. to 500xc2x0 C., namely prior to the start of combustion of vinyl chloride-based plastics, dioxins are formed and the dioxins formed in that former stage of incineration adhere to the ash, namely residue left after burning, formed in the later stage.
Dioxins adhere to the incineration ash and, thus, soot itself contains dioxins. Such soot settles on the surroundings of the refuse incineration plant site, causing a social problem.
Soot containing dioxins adhering to and entrained by the incineration ash has a relatively heavy weight because it contains chlorine compounds. The soot does not diffuse far away but rather falls down on the surroundings of the refuse incineration plant site, resulting, day by day, in accumulation thereof to an increased concentration.
Dioxins are also called environmental hormones or endocrine disrupters. Namely, they may cause mutation of certain genes of animals and plants and, according to some theories, they may cause cancer in humans.
From the viewpoint of natural environment protection and environmental pollution prevention, it has recently been recommended that the combustion temperature in the refuse incinerator should be raised to about 1,500xc2x0 C., since dioxins are formed at refuse incinerator combustion temperatures of 800xc2x0 C. and below.
However, for raising the refuse incinerator combustion temperature from 800xc2x0 C. to about 1,500xc2x0 C., it is necessary to build incinerators newly because the existing refuse incinerators are not fire-resistant at such a high temperature.
The cost of newly building an incinerator is immense. An incinerator with a capacity of 500,000 tons per day, for instance, will cost about 5 billion yen, presenting a difficult problem to local governments.
Refuse is discharged daily and, when it is incinerated in conventional refuse incinerators, dioxins are generated. Citizens good sense is thus expected so that plastics and like packaging materials may not be mixed with raw refuse. As a matter of fact, however, such expectation by itself has failed to lead to any ultimate solution.
A refuse incinerator where the combustion temperature can be raised from 800xc2x0 C. to about 1,000xc2x0 C. may be constructed at a reduced cost and can discharge harmless smoke alone into the atmosphere through its chimney while removing incineration ash through a water curtain. However, dioxins remain adhering to the dehydrated ash resulting from separation of the muddy water composed of water-entrapped ash and water into ash and water by means of a dehydrator, and an extra cost is required for the treatment of that ash and, after all, no complete removal of the toxicity of dioxins has been realized.
The present invention is to solve the above problems and provide a process and apparatus for eliminating dioxins almost completely from soot containing dioxins adhering to incineration ash in a simple and easy manner and at low cost.
Another object of the invention is to eliminate dioxins while causing refuse incinerator chimneys to emit dioxin-free harmless smoke alone without newly constructing incinerators themselves.
To accomplish the above objects, the invention, in a first aspect thereof, provides a process for eliminating dioxins, which are generated from a refuse incinerator and have a structure such that two unit substances having a benzene nucleus are bound together via one or two oxygen atoms and at least one chlorine atom is bound to at least one benzene nucleus, from a mixed gas composed of smoke and ash carrying dioxins adhering thereto.
The above mixed gas is passed through a particulate matter separator, where the mixed gas passes through a water curtain. The dioxin-entraining ash is taken up in water and only harmless smoke thus separated alone is discharged into the atmosphere. The resulting mixture of water and dioxin-carrying ash is separated into dioxin-carrying ash and water in a separator. The dioxin-carrying ash separated in the above manner is washed with washing water, whereby the oleaginous dioxins entrained by the ash are eluted to give an emulsion-like oil-water mixture. The resulting mixture is separated, by means of a separator, into ash, which is now dioxin-free, and the emulsion-like oil-water mixture in which dioxins are dissolved. The dioxin-free ash is washed first with acidified water, then with alkaline washing water and further with neutral water, to give moist ash and aqueous washings; the ash is thus recovered. Separately, the water separated from the above-mentioned dioxin-carrying ash, the dioxin-containing emulsion-like oil-water mixture and the above washings are combined and admixed with a pH adjusting solution, to give a liquid mixture. This liquid mixture is electrolyzed in a primary electrolyzer equipped with an anode and a cathode, whereby dioxin components are decomposed into unit benzene nucleus substances, and a hydrogen atom(s) is(are) substituted for the chlorine atom(s) on the unit benzene nucleus substances. A harmless aromatic oil fraction and gaseous chlorine are formed in the upper layer and the aromatic oil fraction and gaseous chlorine are respectively separated and recovered. The dilute liquid mixture remaining after recovery of the aromatic oil fraction and gaseous chlorine and now mainly consisting of water is transferred to a secondary electrolyzer equipped with a metal anode and a metal cathode and electrolyzed therein to generate hydrogen at the cathode as a result of electrolysis of water and a metal hydroxide at the anode. The remaining trace amounts of dioxin components are decomposed into unit benzene nucleus substances, and a hydrogen atom(s) is(are) substituted for the chlorine atom(s) on the unit benzene nucleus substances, to give a harmless aromatic oil fraction and a trace amount of gaseous chlorine in the upper layer, and the harmless liquid aromatic oil fraction is separated and recovered. The above metal hydroxide takes up those substances that are contained in the dilute liquid mixture as w ell as the above-mentioned trace amount of gaseous chlorine and precipitates in the bottom layer. This precipitate is recovered and the middle layer electrolysis liquid is discharged.
In a second aspect, the invention, which is to achieve the objects mentioned above, provides an apparatus for eliminating dioxins, which are generated from a refuse incinerator and have a structure such that two unit substances having a benzene nucleus are bound together via one or two oxygen atoms and at least one chlorine atom is bound to at least one benzene nucleus, from a mixed gas composed of smoke and ash carrying dioxins adhering thereto. The above mixed gas is passed through a particulate matter separator, where the mixed gas passes through a water curtain. The dioxin-entraining ash is taken up in water and only the harmless smoke thus separated is discharged into the atmosphere. The resulting mixture of water and dioxin-carrying ash is sent to a first separator which separates the mixture into dioxin-carrying ash and water by centrifugal force. The dioxin-carrying ash thus separated is washed with washing water in a first tank to thereby elute the oleaginous dioxins entrained by the ash to give an emulsion-like oil-water mixture. The resulting mixture of the dioxin-free ash and the emulsion-like oil-water mixture containing dioxins dissolved therein is sent to a second separator and separated from each other by centrifugal force. Repeatedly using the first tank and second separator in that order, the dioxin-free ash is washed in sequence with acidified water, alkaline washing water and neutral water, each time followed by dehydration. The water separated from the above-mentioned dioxin-carrying ash, the dioxin-containing emulsion-like oil-water mixture and the respective washings are combined into an oil-water tank and admixed with a pH adjusting solution, to give a liquid mixture. This liquid mixture is transferred to a primary electrolyzer equipped with an anode and a cathode, and electrolyzed therein, whereby dioxin components are decomposed into unit benzene nucleus substances and a hydrogen atom(s) is(are) substituted for the chlorine atom(s) on the unit benzene nucleus substances. A harmless aromatic oil fraction and gaseous chlorine are formed in the upper layer. The aromatic oil fraction and gaseous chlorine are respectively separated and recovered by first and second recovery means. The dilute liquid mixture remaining after recovery of the aroma tic oil fraction and gaseous chlorine and now mainly consisting of water is transferred to a secondary electrolyzer equipped with an anode and a cathode and electrolyzed therein to generate hydrogen at the cathode as a result of electrolysis of water and a metal hydroxide at the anode; the remaining trace amounts of dioxin components are decomposed into unit benzene nucleus substances and a hydrogen atom(s) is(are) substituted for the chlorine atom(s) on the unit benzene nucleus substances, to give a harmless aromatic oil fraction and a trace amount of gaseous chlorine in the upper layer. The above metal hydroxide takes up the trace amount of substances that are still contained in the dilute liquid mixture as well as the above-mentioned trace amount of gaseous chlorine and precipitates in the bottom layer. The light aromatic oil fraction, now harmless, and the above metal hydroxide are separated and recovered by third and fourth recovery means, and the middle layer electrolysis liquid is discharged.